1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an electrophotographic image forming apparatus employing a one-property contact developing system, and a process cartridge used with the electrophotographic image forming apparatus.
2. Description of the Related Art
A general electrophotographic image forming apparatus such as a laser printer, a copier or a facsimile, projects a laser beam to form an electrostatic latent image on a photoconductive drum, and develops the electrostatic latent image into a visible image by supplying a toner thereto using a developing system. The visible image which is developed on the photoconductive drum is conveyed to a printing paper, which passes through a transfer nip defined between the photoconductive drum and a transfer roller, and then is developed on the printing paper at a fusing unit by heat and pressure. Thus, a printing operation is completed.
The above developing system which develops the electrostatic latent image into the visible image by supplying the toner to the photoconductive drum can be divided into a contact developing system and a non-contact developing system. The contact developing system uses a one-property toner while the non-contact developing system uses a two-property toner. The contact developing system is quieter and more power-saving than the non-contact developing system. Further, the contact developing system has advantages such as having a smaller cartridge and a higher resolution.
However, the quantity of the one-property toner which is moved to the photoconductive drum to develop an electrostatic latent image in the contact system is greatly influenced by a contacting area (hereinafter, referred to as ‘developing nip’) of the photoconductive drum and developing roller. Therefore, when the developing nip between the photoconductive drum and the developing roller is sufficient, a printed image of a good quality can be obtained even in the contact developing system. According to an experiment, a substantially allowable range of developing nip, that is, an overlapping outer circumference of the developing roller and the photoconductive drum on a line connecting the centers thereof, in the contact developing system is +0.05 mm˜+0.1 mm. When developing nip area is beyond the allowable range, various problems may occur on a printed image.
Accordingly, to prevent the problems, an integral process cartridge has been used, in which a toner supply unit storing and supplying the toner, and a developing unit forming and developing an image are integrally formed.
An example of such an integral process cartridge is illustrated in FIG. 1. Referring to FIG. 1, in the integral process cartridge, a developing unit and a toner supplying unit are mounted in a cartridge housing 10. In the cartridge housing 10, a cleaning blade, an electrifying roller 12, and a developing roller 13 are mounted in order around the photoconductive drum 11. In addition, in the cartridge housing 10, a waste toner chamber 10a is disposed on one side of the photoconductive drum 11, and a toner chamber 10b is disposed on the other side of the photoconductive drum 11.
According to the above structure of the integral process cartridge, the photoconductive drum 11 is electrified by the electrifying roller 12, and then an electrostatic latent image is formed on a surface of the photoconductive drum 11 by a predetermined laser beam 14 which is projected from a laser scan unit (not shown). Then, as the photoconductive drum 11 rotates, the electrostatic latent image is developed into a predetermined visible image by the toner which is conveyed by the developing roller 13. The visible image is transferred onto a printing paper P which passes between the photoconductive drum 11 and the transfer roller 20.
Accordingly, since the developing nip between the developing roller 13 and the photoconductive drum 11 is maintained by the cartridge housing 10 regularly in an electrophotographic image forming apparatus using the integral process cartridge, an image of good quality is obtained.
However, a defect of the integral process cartridge is having to replace the whole process cartridge when only the toner in the toner supplying unit is used up while the developing unit including the photoconductive drum is still usable. As a result, the integral process cartridge is more wasteful and costly.